Though usually constructed by shipyards, floating drydocks are civil engineering structures. Dry docks that float have one important advantage over their non-floating relatives — they make possible launching a newly built ship in a completely controlled manner. When the dry dock is the launching platform the ship can be assembled in a horizontal attitude in a dry environment well above sea level and then launched at the attitude assumed by the vessel when free floating. Similarly, a ship can be retrieved by the dry dock in the same manner. Thus, no undue stresses or strains are placed on the vessel or the dry dock.